For Glass Surfaces I use this safety blade, these blades come individually wrapped and have only one sharp edge.
The Single Edge Metal Blades Are Available On eBay
For plastic to potentially scratch able surfaces the I use this plastic blade, these can be purchased with a holder or use on their own. These are really useful to have around for a variety of jobs.
The Plastic Razor Blades Are Available On eBay
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Thanks fixed1t