I have used home made wax/oil proofing on other items, like packs, bedrolls, tote bags and hats. Google it but it is basically 1/3 Beeswax, 1/3 Boiled Linseed Oil, 1/3 Turpentine. I would go easy and use a fairly dilute mix. The roof can get three coats and the sides two. But if you try with one heavy coat it will be a sticky mess. Set it up in the sun and a heat gun helps to absorb the wax faster. It will need to air out for a week or so. Test it after curing with a garden hose.
But as others have written a poly or separate treated tarp as a fly is better in many ways, especially if you use the tent in warmer weather. The untreated fabric breathes much better. A cheap canvas tarp could be treated and it will save adding weight to the main tent. The double roof is better for both heat and inclement weather. This is the standard in Africa.
