I hope I can be of some help in technical nature, rather than in botanical ways, since I never tried this.
When treating with radiation best results could be archieved with gamma rays, but technically they are complicated to shield (handle) so I would not recommend. X-rays can easily be produced electrically and shielded with reasonable effort, they are pretty much DIY thats why they were discovered and reproduced so long ago. They have proved to be easily capable to change crops characteristics. I can not advise on how much mortality one wants to aim at to have sufficient changes in the viable seeds (and basically less effort growing and selecting). UV Rays will sadly not penetrate deep enough.
(I think I should mention heat is also known to manipulate genes at random but the line between fatal and changed characteristic is very narrow.
The other option I'd consider are chemical treatments, the number of chemical compounds capable of making theese changes (some to frigthening extend, very fast and efficient) is virtually unlimited but many are restricted and one could argue this is within reason. When trying to cause mutation chemically one should take the way of action into consideration, some chemicals for example are faulty build into the DNA during replication (wich doesnt really happen in seeds so its not applicable). Others can recommect the whole "DNA grid" and dont need replication.
Lastly Viruses should be mentioned, they can be build into DNA but I would really not recommend them, let alone know who would quickly hand the right one over to anyone.
Id say considerably safe and obtainable are Nitrosamines and ordinary petrochemical products (in both cases the Vapours should do). Exposure will sure be to be determined in experiments. Smoke will possibly contain benzopyrene amongst other nasty stuff, cold smoking seeds might help. Hydrogen Peroxide in higher doses should do. Benzene and Formaldehyde could do a nice job. Many other chemicals I could think of will possibly be very overkill.
Another idea: in my area and propably many others Colchicum Autumnale can be found. It is known to be "highly toxic" and this is due to a mutagene (1.8% in petals). It works on plants but I cant explain why not on the plant itself. The poison messes with the mitosis. In plants it has been used before to restore fertility in plants that lost the ability to reproduce by crossbreeding. It will create polyploides - again its a narrow area in wich we get viable polyploides, not untreated and not dead seeds. Be aware it will also want to make you polyploide.
Hope you stay safe
I can advise on producing ionising radiation or nitrosamines.