listen to rivet about the wheels - you have probably already found out about them, but just in case you didn't find out - pay attention!
ok - here are three more that will be very inexpensive and help out a lot.
go to the hardware store and get some dryer vent tape. they make stuff for wood burning stove pipe, but it is awfully expensive. dryer vent tape should be less expensive and will work just as well. use it to seal off the gigantic holes at each end of the smoke chamber - the ones that must have been cut out to accomodate a rotisserie? if you ever need to use them, just rip the tape off and put more on when you're done using them. tape both inside and outside.
the second is just as easy and will maximize a lot of your SnP's potential. go to a muffler shop. take the base of your chimney (the part that the chimney fits on top of) with you. tell the muffler man that you want a piece of new exhaust pipe cut to nine and a half inches long, then ask him to cut a 45-degree angle into one end. this will make the "long" part nine and a half inches long and the "short" part 7 and a half inches long. ask him to swage (or swedge) the exhaust pipe into the bottom of the base of the chimney (not the chimney itself). what you will end up with is a pipe that sticks down into your smoking chamber to stop flush with the grates. the 45-degree angle must be rotated so that when you close the grate, the bottom is flush with the grate. here are a couple of pix, shamelessly borrowed from rivet:
if you look closely, you can also see the dryer vent tape closing off the rotisserrie holes.
by some weird design eccentricity, the chimney pipe of the SnP is between 2.5 and three inches wide. this means that you will need to use either a 2.5 inch or 4 inch exhaust pipe. three inch would probably be better because it would allow a bit more draw, but the diameter of the chimney ultimately determines the dray, and it is not quite ththree inches, so if you have to use 2.5 for whatever reason, it shouldn't be a problem that i am aware of. if i am wrong about this, anyone is free to correct me.
the good news is that this tialpipe business will cost you between four and six dollars, most likely! :)
thirdly, a good mod to do is to build a charcoal basket; i will let rivet give you the specifics on that, but until you get a fire basket, you can substitute by opening up your firebox, moving the bottom grates to the top (setting them on the top grate) and building your fires on the top grate instead of the bottom grate. this will allow you to have plenty of room for your fire to breathe and will not let ash choke it out too bad, as there will be plenty of clearance. the point of moving the bottom grate to the top and setting the grate on top of the top grate is so that smaller pieces of charcoal (either form lump charcoal or as the briquettes burn down) will not fall through the grate so quickly.
finally, think about some designs for a baffle, or manifold. i'll let rivet go into this as he is the master of this particular mod.
good luck and enjoy!