You could use a second hand Naim (older) types, but they don't come cheaply.
On Aliba you can also buy half and full width cases in black. Small ones are +/- £60,- and the bigger ones about £100,-. So not cheap. Also don't forget the transportation costs, because of the weight its expensive.
A good alternative is buy aluminium profile. You can get 200 mm x 80 mm profile in any length you want. For a Naim case 300 mm length is best. I believe you can also get 200 mm x 100 mm. They can cut it to length for you. Costs for this is low. For the inside you make a sled (like Naim) from a piece of aluminium sheet that you bend on both sides creating a front and back. If you can't do this yourself, you have to ask the seller of aluminium can do this, some do, but it will cost. For the outside colour you have to spraypaint it. This is all DIY, so it's up to you.