Danelectro Rocky Road is very small, cheap and does a nice job.
everybody's of course got their own opinions on this subject, however i had the same question about 2 years ago, and since then, just about every "leslie" box on the market has sat on my board for various amounts of time. i've ranked them and heres my opinions on each...
1. Korg G4 - the best of the bunch. however, we all know pedal board real estate is at a premuim and this guy take up a lot. i keep mine around to record with. also, i don't believe they make it anymore so...it cost me...
2. Hughes and Kettner Rotosphere - also very very authentic. but again, huge pedal and i bought it just after i got my g4 and it lost the shootout....so off to the buy and sell forum it went....
3. Strymon Lex - this sits on my board currently. very rich and thick sounding.. smaller and less expensive as the top two.
4. DLS Rotism - this guys cool, because of the multiple trim pots on the inside. also by far has the most dials and variance out of all the leslie pedals i've tried. also takes the most time to monkey with to find "your" sound.
5. Boss RT-20 Rotary - this one sounded a bit too fake for me (like chorus trying to do a leslie). it also didn't get along with the rest of my fuzz chain. but by itself plugged into my Dr. Z it sounded ok.
6. Fulltone Deja-Vibe - this has been on my board for awhile and i've noticed a can get a decent "fast" rotary sound out of it if i set it on vibrato and turn the speed all the way up......just sayin....
7. Mojo Hand Gyro - this pedal i thought at first sounded terrific. it's downfall was going to see gov't mule a couple days after i got it. after hearing warren's leslie tone it got the boot....and i went back to my g4....lol
8. Option 5 destination rotation single - i bought this pedal because of it's smaller size. but the tone i got was terrible. it squashed my signal, and sounded like a leslie recorded in a garbage can....
again....all my opinion. just figured i'd share since i've been on this same leslie trip forever. if you have the board room and the cash (short of buying an actual leslie) get a korg g4 or a hughes and kettner. if your short on space get a strymon.... hope this helps.