Try getting a small roll of foil/bubble/foil (reflectix) insulation for your bucket. It costs about 12 bucks. Cut it to size, attach it with foil tape, cover the top with your kettle lid, and it will hold mash temperature almost as good as a cooler.
Use the money you save from not fabricating a cooler mashtun and get a 8.5 gallon enamel canning kettle and an immersion chiller and you can do 5 gallon all grain batches with up to 12 lbs of grain. You can use your 5 gallon kettle to mash - in and add heat to get to mash temps. You can do step mashes if you want to with this method. Dump the mash into your bottling bucket lauter tun, rinse out the 5 gallon pot, fill it up with water and use it as a hot liquor tank. I use a pyrex measuring cup to start the mash transfer and then to sparge. Put the bucket on a chair and drain it into the 8.5 gallon kettle. It won't take much longer than doing partial mashes. I have used this "ghettobrew" setup for several batches and it works well - I average 75% efficiency.
Here is a link to where I got my kettle...
http://www.amazon.com/Granite-Ware-34-Q ... 579&sr=8-1Immersion chillers can be had from several places - I got mine from B3.
Think about it. You
can do 5 gallon all grain batches on your kitchen stove fairly easily like this. Once you get the hang of all grain, then get a larger cooler of 10 gallon or so and you can step up to 10 gallon batches with a 15 gallon kettle and propane burner.
This is an easy and low-cost way I made the transition to full boil all grain batches. I am now looking at stepping up to 10 gallon batches with a burner and larger kettle. All of the advice I have gotten has said "don't bother with a 5 gallon cooler, get a bigger one and do 10 gallon batches instead."