Two of the most common rifles of the Rocky Mtn fur trade era were brass mounted late era Lancaster rifles built by makers such as Dickert (pre-1830) and Henry (his rifles were first sold to the western trade in 1826), who were just two of the main suppliers to the fur trade companies.
One of the other widely used rifles was the iron mounted Southern Mtn Rifles.
Both are available in kit form from Muzzle Loading Builders supply as well as other dealers.
For the time period swamped barrels would have been the most common, although tapered and straight barrels started being used more by around 1830. English style locks such as the L & R Late English or the Chambers Late Ketland which is one of the best locks on the market for this period and type of rifles, were teh most common.
Another option is a Leman kit, in essence a late Lancaster style, just don't use the typical Leman "capbox" replace it with a late Dickert or Henry style patchbox and proper flint lock and call it a Lancaster.
For a plain jane rifle, go with a So Mtn Rifle - for one of the best examples see Dennis Galzener's Gillespie Rifles page
http://home.att.net/~t.glazener/wsb/html/view.cgi-home.html-.html
Calibers: while 50 and 54 were widely used in the West 45's were not unknown; in fact the Henry New English Model, first offered in 1834 was of that caliber to begin with and several western fur trade purchase orders from that time list it as a caliber.
IMO either style rifle with a 42" 50 calber B weight swamped barrel (available from several makers and a very common length for the period), a plain maple stock, single or double triggers - your choice - just match to the correct size trigger guard, with the proper lock (see my above suggestions, the Germanic style locks are not "correct" for this period) and furniture to match - brass for the late Lancaster and iron for the So Mtn - will best fit what you are looking for. Much depends on what time period you are looking at - the western fur trade should really be broken up into at least three periods when it comes to equipment, with some over lap of course.
FWIW there is an outfit making Henry rifles - English models as well as Lancasters - but they are not available in kit form at this time. Also Dimick didn't arrive in St Louis until 1849, past the classic Rocky Mtn fur trade era.
But you might check with Don Stith,
www.donstith.com , he was working on a proper Henry Lancaster style kit - don't know where he stands at this time.