i'm unable to find cattleguard listed as a single word anywhere, but I think it ought to be an acceptable spelling
I would not spell it as one word because there are many variants of "Cattle Guard" and they are all spelled in two words. "Cattle Grid," "Stock Grid," "Vehicle Pass," "Texas Gate" are all phrases used in cattle country in various countries.
You are certainly correct in observing that cattle guard is the preferred spelling, as confirmed by both Merriam-Webster's Eleventh Collegiate Dictionary (2003) and The American Heritage Dictionary of the English Language (2010)—neither of which lists cattleguard as an approved variant spelling.
Nevertheless, cattleguard clearly is a variant spelling, as both its lengthy history of use and its track record as reported by Ngram indicate. The Ngram chart for cattle guard (blue line) versus cattleguard (red line) for the period 1850–2008 looks like this:
The form cattle guard is clearly more common—although it bears observing that a number of the Google Books matches nineteenth-century matches for cattle guard actually involve instances of cattle-guard (hyphenated), and that almost all of the nineteenth-century matches for cattleguard are instances where cattle- appears at the end of one line and guard at the beginning of the next.
There are, nevertheless plenty of instances where cattleguard appears as one word on a single line. The earliest of these may by an error, since it appears in a header to a court decision that otherwise consistently uses the form cattle guard. From "Thompson v. The Grand Trunk Railway Company" in Queen's Bench and Practice Court Reports (1876), a Canadian case:
Railway crossing—Cattleguard encroaching on highway—Accident—Liability—Contributory negigence.
But Justice Samuel Blatchford of the United States Supreme Court spelled the term cattleguard six times in Chicago, Milwaukee & St. Paul R. Co. v. Artery (December 22, 1890) in The American and English Railroad Cases: A Collection of All the Railroad Cases in the Courts of Last Resort in America and England, volume 44 (1891) including these two instances:
The petition was afterwards amended by alleging further ... that the cattleguard was made of three-cornered pieces of wood, placed negligently on top of the ties, across the track, instead of lengthwise, and some of the three-cornered pieces stood higher than the surface of the rail, of which fact the plaintiff was not then aware; and that, by reason of such negligent construction of the cattleguard, the speed of the handcar, and the dangerous and tiresome position in which the defendant placed the plaintiff, he was injured either by his foot or feet coming in contact with the rail or the three-cornered pieces, or by the shovel getting caught on the rail or on such pieces,or by all of such circumstances.
And Engineering News (November 17, 1892) devotes an article to "The New Standard Cattleguard," which begins as follows:
We have from time to time illustrated a number of forms of surface cattleguards, and this week present a cut of new iron cattleguard designed by Mr. B. Welhaupter, C. E., manager of the Standard Cattleguard, Chicago. It is a modification of and improvement upon his original standard cattleguard No. 1, illustrated in our issue of Jan. 9, 1892, and has been designed to meet the requirements of some roads for a cheap but efficient cattleguard.
Many more-recent publications use the single-word spelling, too. For example, from U.S. Department of the Interior, Bureau of Land Management, Roswell Resource Area Proposed Resource Management Plan/Final Environmental Impact Statement [and] Carlsbad Resource Area Proposed Resource Management Plan/Final Environmental Impact Statement, volume 2 (January 1997):
Where used, all cattleguard grids and foundation designs and construction shall meet the American Association of State Highway and Transportation Officials (AASHTO) Load Rating H-20, although AASHTO U-80 rated grids shall be required where heavy loads (exceeding H-20 loading), are anticipated (See BLM standard drawings for cattleguards). Cattleguard grid length shall not be less than 8 feet and width not less than 14 feet. A wire gate (16-foot minimum width) will be provided on one side of the cattleguard unless requested otherwise by the surface user.
These examples show that the spelling cattleguard has existed for more than a century. Whether you can prevail upon a publisher (or academic institution) to let you spell the term that way depends on whether you have a publisher, whether the publisher generally follows a particular dictionary's spelling preferences, and whether the publisher is willing to make exceptions in cases like this one. If you're on you own, I think you should feel free to spell the term however you like, notwithstanding the fact that the authorities favor cattle guard. After all, no one is going to find it harder to make sense of cattleguard than of cattle guard.