There is no prescriptive rules that cover where to end katakana section when you write mimetic words, interjections and slangs in katakana (because they are colloquialisms anyway), so we don't have the "right" answer. It mostly depends on personal interpretation: that whether you want katakana-ify the concept or the word when you do, and where is ...
One option that’s rather difficult to type in normal font is あ゙ (あ with ゛ (dakuten)). Although not an official letter, it’s been used in manga to express approximately the effect you are looking for. Wikipedia
Paddle paddle. Probably referring to hagoita used in hanetsuki (Japanese badminton), a traditional Japanese New Year's game often played by girls.
And/or staying with the movement theme as in wading through Tsukimihara U.
Good article on the topic: http://nihonshock.com/2013/04/japanese-onomatopoeia/
Basically, most giongo/gitaigo can take four forms with basically the same meaning
Combination of the previous two (にっこり)
Probably the only difference in meaning is described by Hikari in the other answer: the ...