One of the more well-known 1/12 scaled toy companions to the MMM line are the Colorforms Outer Space Men. Here are some additional items you may want to consider adding to your MMM collection:
Here's a TONKA Crater Crawler from the Moon Race era. It has been slightly modified to accept a Matt Mason scale figure.
This toy appeared at the same time Mattel was marketing the MMM toy line, clearly to appeal to those interested in the Moon landings. While most Tonka trucks are scaled for 3&1/2 to 4 inch figures, I could not resist trying to fit a MMM astronaut into one. The blue/white color scheme, and simply the name "Crater Crawler", were too reminiscent of the Mattel toys. You can periodically find these crawlers on eBay.
This project will require permanently damaging part of a vintage toy...so this is not for the faint-hearted*.
The modification requires you to remove the inner plastic driver's compartment, and cut out the bottom floor, and the far right and left armrests. That's really all there is to it. This will give the figure the needed leg and hip room to fit into the seat. You can even squeeze TWO figures into the seats. I also added two handle-extension (thin plastic tubing) to allow the MMM figure to reach the controls.
As you can see, the plastic canopy will not fit on top of the driver compartment as originally intended, but it can easily be placed on the rear section, to allow transportation of items that might require a sealed containment.
*If you really don't want to damage your crawler, try using a damaged MMM figure missing his lower legs, or even just missing the boots.
A line of toys under the name "Space Voyagers" includes this model of the Lunar Rover. It is made for 5 inch "Ultimate Action" astronaut figures, but still fits nicely with the MMM figures. I've got a blue Leadworks Jeff and a green repainted Sarge driving this one. The seatbacks are positioned quite a bit to the rear, so the figure has to really lean back to settle in. That's okay for the leadworks fellows, as their hip joints really don't bend that well anyway. The green sarge is sitting up more straight, but is not touching the seat back.
I've also rotated the control panel by 180 degrees to the rear, so that the controls are near the astronauts' hands. The solar panel also rotates, and the wheels turn.
You can buy these rovers at this website, or off eBay. They really look nice with the Matt figures. I think the Matts actually look better than the figures for which the toy was meant.
I picked up this 1/12 scale resin replica of the 2001: A Space Odyssey EVA pod off of eBay. It is definately a non-professional "cottage industry" product, and I do not know how many were made. This particular kit has a removable side panel allowing access to the interior. The red and yellow leadworks MMM copies are re-enacting one of the most tense scenes from the movie.
The kit was made by a fan of the Madelman series of figures, and meant to be displayed with their 2001 astronauts. But it fits perfectly for the MMM line as well. I have found one collector's website with a similar pod on display at this site http://www.alacarga.com/contenidos/madelman/jp65.html.
If you want to build your own, a more accurate resin kit has been announced by Atomic City models. It will have a fully detailed interior and you can probably add interior lighting if you are crafty. You can check for availability here http://p197.ezboard.com/batomiccity.
Anyway, the lesson is keep checking eBay...you never know what you may find that's suitable for your MMM collection.