If you do any wood working you probably have a circular/table saw and a drill. Get yourself a pocket hole jig, screws and construction adhesive.
Get some PT 2x6s. Cut one edge so that it is square to the face. Cut the 2x6 into about 10-12" pieces. Edge glue and pocket hole screw these into flat "plates". Now you have light weight spacers that you can stack. If you are concerned about them shifting, stack up 3 or 4 together and drill through them. Place a loose fitting wood dowel through them.
If you want a wider base, put 3 together.