What if you don't have a Martian controller?

The slaver tripod has a head slightly larger than an assault tripod, but with squatty legs sort of like a grenadier.
If you can source an assault tripod, you can cut down the straight bit of the legs, just below the "knee," then cut the tentacles off of the "arms" to make a passable substitute.

The rules don't 'officially' have a 'controller upgrade.' That said there is no reason that you can't come up with your own house rule such as 25 points to make any tripod into a controller. Alternatively you can designate any of your tripods into a slaver instead by just designating it and perhaps marking it differently. Pay the slaver points and give it slaver stats.

As for proxies, the best alternative I can think of is the Small Martian Walker at Rebel Minis: www.rebelminis.com/miwa.html for $15 each.
at 2" tall (+1-2" for the antennae) I use it for my Scientist, which is a controller and although moving slower (same speed as the lobototons), it also gives one of your Assault tripods a random special ability.  Tactically I use my Slaver up front driving the drones and lobototons with the Scientist bringing up the rear to pick up stragglers and then take the place of the Slaver when it is inevitably destroyed.  Note that solitary Slavers tend to draw lots of fire since taking them out also immobilizes all the Martian infantry, essentially taking them out of the fight as well.  

Rebel Minis also has a Large Martian Walker www.rebelminis.com/martianwalker.html which at 4" tall would be more comparable height to the AQMF tripods.  Although it is twice the price, at $30 each, it is a big heavy lump of metal.  This is somewhat challenging to glue together the legs (really requires pinning the legs when assembling) and can cause additional problems when transporting, just imagine dropping a box of fragile plastic tripods with a brick in it and you see what I mean.  I have one but these are the main reasons I haven't put mine together and started using it.  

Here are for comparison, all are AQMF unless noted; Assault 5 3/4", Scout 4 3/4", Slaver 3 1/2", Rebel Minis Small Walker 2" (not counting antennae), Grenadier 3". Note that there are two 18mm AQMF humans on the Slaver base for size comparison.

BTW I am currently working on a 15mm battle of Medhia Beach/Port Lyautey where C Co, 70th Tank Bn landed 12 M5 light tanks off of landing craft in Morocco and fought off Vichy French Renault R35 tanks during Operation Torch.

Wow, really like the small walker thanks for the link! Definitely going to see about picking up one. Thanks

Yes love my find on those last year - used for basis of many alternative tripods, including my illfated AA tripod etc. Those top tentacles can be use for other machines as well and won't be missed on the machine pictured. The big one stands in as an alternative Royal Scientist - with a new weapon type - I'll have to post the stats for that one. TY for the pic and comparisons wisercj.