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I think that this is a not difficult to be make, all is depend from your skills.
If you want good and compact power supply you must use switching mode PS.
See link below, this is a very good SMPS controllers, in the site you an find soft for calculation, but you must know that PSPU quality is directly connected to the transformer quality.
And ... i not sure that home made power supply will be cheaper from original, but if you want to try this IS is good solution.
I think a simple linear power supply will be just what you need and can build without being involved in quite tricky SMPS issues ..
Take 16V transformer rated for at least 5A, 10A bridge and 4,700-10,000µF/25V electrolytic capacitor to generate ≈21V unregulated voltage.
Then build circuit based on LM7815, or LM317 in current boost configuration (see picture below) ..
And don't forget about reasonble size heat sink for PNP power transistor (2N2955 or similar) ..
If you have a DC voltage another good IS is a LM2575 series, in your case LM2678, you can see the attached files. But this can use only if you have DC. You need have a transformer and diode bridge with capacitors before this schem.