I like the Musical Memory Mapper, I can't remember what I used it for, but it worked on MSX1. (Atleast for making 32kb machine into 64kb.)
Also, there are schematics for memory mappers out there that you can make on a protoboard and ebay parts (made one a few months ago with lots of wires.)
So it's not worth making such a memory yourself ...
And you can give me some links where to buy such a cart
If I had to buy a new one today, I'd check the Flashjacks, it has RAM and many many more stuff.
But still not available i think? -> Flashjacks
If you don't need so many features, a good option is a Megaflash SD reader. Appart from SD reader and flash they can come optionally with at least 512 Kb of RAM I think. -> Cartridge Shop
As they say, there are more (tho i didn't use them), like
Music Memory Mapper -> MMM
the Carnivore -> Carnivore
yea flashjacks.....
MMM - what to eat it with?