You can insulate the tank by covering the bottom of the tank (on the outside), the back, and maybe one or both sides with anything that heat has a hard time passing through. If you plan to use a sump, on e possible trick is to use a plastic ice chest, instead of a glass or acrylic box, as your sump.
For the tank, I've tried a few options:
1) Styrofoam brand insulation board. You can find it at home improvement stores or lumber yards in various thicknesses. I usually buy 1" thick, use 1 layer for one side of the tank, and one or two layers for the back. I use 2 layers for the bottom of the tank. You can measure and cut it to size with a hand saw, or power saw. It's blue, and not very attractive, so I usually go to the fabric store and buy a couple yards of black vinyl (cheap) to wrap each cut piece of Styrofoam in, like a present, using black duct tape. You can even sew the vinyl to make a custom fit cover for a cut piece of Styrofoam. Attach it to the tank back and side(s) using adhesive Velcro (sew the Velcro to the vinyl, and stick it to the glass) or black duct tape. cut a piece to fit the bottom of your tank, and place it between your stand and your tank (it can easily support the weight). If your tank is already full, you can still attach Styrofoam to the exposed parts of the bottom, or to the underside of the wood your tank rests on. Actual Dow Styrofoam brand insulation sheets are rather stiff, and so work well for glass tanks because they don't bow. the walls of acrylic tanks bow out a little, so you'll need to use generic (usually white) styrofoam (not Styrofoam brand) that are a little bit flexible.
2) Fiberglass insulation, like for an attic, or a water heater blanket, or Polyester batting (like they use to make quilts - from the fabric store) You would need to make a cloth cover, like a big pillow case, put a layer of insulation into it, and then find a way to attach it to the sides, back, and bottom of the tank.
The front and maybe one or both sides of the tank are still uncovered, and letting a lot of heat in, but it's a lot harder to insulate transparent surfaces. Using an acrylic tank is a little better than a glass one, but to really insulate transparent sides, you need to glue a 1/4" to 1/2" spacer along the edge and attach a second pane of glass or acrylic, to create a sealed air space between your tank, and the new sheet of glass or acrylic. This is difficult to do correctly, but I've done it, and I've learned a few tricks. I'll write up the method if anyone ever feels ambitious and wants to try it, but that's another (long) post.