Try finding a cheapo guitar effects pedal that handles equalization. There are graphic EQ's ( they have the slides ) and there are parametric, ( which use knobs ).
Either way, you want to take out some of the bass frequencies, and boost the mid range a bit. You're sort of aiming for an AM radio vibe. Just experiment.
You obviously won't fit an effects pedal in your bucket, but, you can gut it, and tuck the necessary bits in a pouch, or maybe in your bucket, depending on how you have your amp system set up. It's cheaper than the popular costume rigs out there, but those systems do sound awesome!
Here's a sample of what they look like. I'm not advertising or endorsing any of these, just using these as a "for instance ".
http://shop.ebay.com/?_from=R40&_trksid=p3907.m38.l1313&_nkw=graphic+eq+pedal&_sacat=See-All-Categories
http://cgi.ebay.com/ARTEC-SE-PEQ-PARAMETRIC-EQ-Guitar-Effect-Pedal-CONTROL_W0QQitemZ120437093000QQcmdZViewItemQQptZGuitar_Accessories?hash=item1c0a9c3288&_trksid=p3286.c0.m14&_trkparms=65:1|66:2|39:1|240:1318|301:1|293:1|294:50
