This power supply is powerfull and easy to make. I've been using it for a while and i'm very satisfied. It can produce 5A at 30V. All of the parts are available on ebay.
Step 1: Part List
http://www.ebay.com/itm/DP-30V-5A-Constant-Voltage... DC-DC power supply modul
http://www.ebay.com/itm/10Pcs-4mm-Banana-Female-Ja... Banana connectors
http://www.ebay.com/itm/AC-DC-Inverter-Module-110V... AC-DC converter
http://www.ebay.com/itm/IEC-3-Pin-Chassis-Panel-Mo... IEC C14 connector
http://www.ebay.com/itm/Rocker-switch-rocker-switc... ON/OFF Switch
http://www.ebay.com/itm/Half-shells-Casing-Euro-Bo... Plastic housing
Step 2: Housing
When you have all the parts, you can decide their posision on the housing. The easiest way to cut big and sqare holes in the plastic housing is to drill small holes realy close so you get a sqare lined with small holes, than you just push out the sqare and grind the edges.
If you use plastic housing you don't need to ground the housing, but if you use a metal housing you have to connect the middle pin of AC connector (ground pin) to housing for your safty.
Step 3: Connections
Connect parts like on the pictures.