That size could be kinda heavy and unwieldy to drag around on off camber stuff. Maybe with an aluminum frame but that would be pricey. Could build it to break it in two and bolt the halves together. Or build it shorter and plan on piling some snow to lengthen the ramp. Or build it out of 2X4's and or angle iron and use bolts to fasten everything together. That way you could dis-assemble it if needed to get it into tricky spots.
Just frame up the ramp part like a wall with 2'? center's on the 2X4's and screw on some 1/2" or 3/4" plywood. You could use it as a sled to haul the legs and sill's. For uprights you might get by with one 2X4 at each corner, but one or two in the middle would beef it up better. Use 1/2" or 3/8" bolts to tie these into the 2X4's of the ramp. Tie the bottom together with a 2X4 and some simpson strong tie plates screwed in, or maybe better to use some strap iron corner brackets bolted in. Screw in a brace at an angle from corner to corner on this structure. Stay away from nails cuz their gonna wiggle loose pretty quick and you can't tighten them up. Next, bolt a 2X4 to the bottom of the leg structure back to the low end of the ramp, on each side. probably want a couple of cross braces bolted in about 4 feet back from the high end for lateral stability. 1.5" angle iron in an X pattern would probably be easiest to fasten in. Cut away 2" of one flat/side of the angle at each end and bend the tab to the angle needed to bolt to the 2X4. Could use wood with metal straps for this too. Fasten at the center X with a bolt.
To anchor it down you might get away with digging it down deep enough and piling/packing some snow in front. You could extend the front legs down past the base a foot or two as well, but I could see this causing problems and tearing up the ramp or having it flip forward.
You might just get by with a snow anchor at the rear. Use a piece of 2X2' plywood or bigger and build a stout frame with a center stud, fasten a rope in the center through the stud and bury it several feet down a good distance behind the ramp. Angle the plywood so it's flat to the pulling angle and bury it, packing the snow in front of it. Tug on the ramp to set it and see if it holds.
For hauling I'd probably fasten a few ropes or straps to the end of the ramp in the center, about the width of your tunnel apart. Lift the the front of the ramp and strap it fairly tightly to your bumper so it hangs just below an inch or two. That should keep it behind you pretty straight, but not damage your bumper if it hits a stump and tweaks sideways. Might have a guy ride behind holding a rope to help steer it. Should drag ok but will probably work your springs pretty hard. You could try dragging it with a rope just long enough to keep the front up a little (5' or so) but reduce the weight on your sled. Somebody at the back steering should help out whatever you do. Or bolt the long 2X4's flat to the plywood so it tracks better.
this is all probably on the flimsy side, but sort of light and might work??? Might need to do 12" centers on the ramp bed to take the abuse. Start out with 2' and see how it holds. A few vertical braces from the sill to the outside edge of the ramp would be a good idea too. Making something like this strong enough and portable might not go together!!!!!! A welded steel frame would be better but not very portable. A welded aluminum frame would be the best and you could probably manage it in one piece, but to have someone build it for you would cost a few grand or so. Get yourself a Tig welder or rent one? and have at it! Awww, just keep building snow ramps
