Well just because you operate away from a "real" airport does not mean that you can't go find an airport with a compass rose and do your adjustments. Or if you know the true bearings of prominent landmarks from a spot on the location that you fly from, and you can beg, borrow, or rent an inexpensive surveyor's transit, you can draw your own compass rose. The transit will have a 360 degree protractor as part of its base. You point the transit's telescope at your known landmark and turn the base until the known bearing number matches the direction the transit is pointing. Then you rotate the optics to each of the cardinal points listed on your compass card and have a buddy stand out there with stick and mark or stake that spot. Once you have all of the stakes in place, you tow or taxi your airplane to put the on board compass over the spot that the transit was at, and you follow the procedure in AC43-13.

If you bring some beer for your friends and helpers and invite some other airplane owners to swing their compasses too, you can make a party out of it and everyone's navigation might get a little better too.

You could also invent a procedure where you fly a series of GPS courses and write down the deviations as you fly, and then guess the adjustments when you are back on the ground, but that juggling is likely more difficult to do.

As you note, compasses are important. You haven't lived until you have been out in strange mountains, with nothing but a compass and a sectional chart and your wrist watch, in a bare VFR equipped airplane, with weather coming down, following that compass to a strange airport.

Best of luck,

Wes
N78PS