the safest easiest method
and least expensive method is to use 2 sections of pallet racking. Ideally you need a total of 4 ten or twelve foot uprights and 4 ten foot crossbeams. All can usually be bought as surplus for around 200. Assemble the units so they sit beside each other and pull trailer through and under. The cross beams are going over boat from side to side. Jack up trailer and boat as previously explained and suspend boat with mutiple rachet straps down and under boat. The hook end of straps will usually fit nicely up high in unused adjustment holes on uprights. Release jacks and lower trailer . Roll out trailer. When not working on centerboard, use a horizoatal peice of 4x4 and a floor jack to give keel some support. By using multiple straps(more than 2) you have larger safety margin and can alternate straps to get all areas painted. Use old rope to lash inner legs of uprights on each side together. This makes it more one unit but still lets it flex to find level. Put old folded bathtowels under straps where they go over hull to deck joint. I have used this method many times and find it far safer to suspend instead of blocking under. On my O'Day 23, I use 6 large rachet straps. Also you can break down this system and store in the garage rafters or similar untill needed again. Good luck