How to drill through pennies?

[mcfonz]

Basing wise my advice would be if you are using two discs to make the lower of the two bigger if poss. So maybe use 20mm for top and 25mm for the bottom and then fill the difference with a slant to it. I find it helps to make it look less chunky, and helps it look better visually.

You could also try adding some small pebbles / gravel to the base to add weight for stability.

[Silent Invader]

How about metal buttons, the sort used on trench coats with 4 holes?

[Vonkluge]

How about metal buttons, the sort used on trench coats with 4 holes?

Ok you have just proved you are "slightly insane"!   

[Silent Invader]

Ok you have just proved you are "slightly insane"!   

 

But they come pre-drilled with 4 holes!

[Dewbakuk]

But they come pre-drilled with 4 holes!

True...

[Major_Gilbear]

Actually, I was going to suggest curtain weights...

I think they would be perfect for what you want as well, not least because they are round and heavy, but are also fairly easy to drill/file.

I buy lead penny weights from Merrick & Day here, and they are perfect for adding a substantial heft to plastic slotta bases.

Depending on how many you need, you could also try looking in any local haberdashers, or even places like EBay. Note that they sell different sizes as well, so if you do some searching, you can get all sorts of diameters and shapes.

[Dewbakuk]

Curtain weights? That would never have occurred to me. I shall look into that, cheers.